The present invention relates to aqueous compositions based on crystalline or semicrystalline polyurethane polymers, processes for the preparation thereof and adhesive systems comprising these aqueous compositions and the use thereof.
The use of aqueous dispersion adhesives has increased greatly in importance in recent years because of their good environmental compatibility. In particular, dispersion adhesives based on polyurethane polymers with crystalline or partly crystalline polyester segments have been able to replace solvent-containing adhesives in a number of industrial uses.
As a rule, aqueous dispersion polymers are processed in this context co-using liquid, hydrophilically modified, predominantly aliphatic polyisocyanates (so-called 2-component processing). However, during the 2-component processing the formation of labile carbamic acid derivatives which dissociate into the amine on which they are based and carbon dioxide occurs between the polyisocyanate and the aqueous phase of the dispersion. The amine released in turn reacts spontaneously with free isocyanate to give urea. The isocyanate required for the crosslinking of the polymer chains of the dispersion polymer is slowly consumed in this manner by the reaction with water. The pot lives, that is to say the maximum processing time of the 2-component adhesive dispersions of a polyurethane dispersion and a hydrophilically modified, predominantly aliphatic isocyanate, depend inter alia on the pH of the formulation and are typically 1-12 hours.
If the 2-component dispersion adhesive is processed after the pot life has elapsed, the amount of isocyanate groups is not sufficient to crosslink the polyurethane polymer completely. The result is then an adhesive bond which does not achieve the required quality with respect to heat resistance.
The reactivity of the polyisocyanate in the aqueous adhesive dispersion should thus be as low as possible in order to ensure a long processing time.
At the same time, however, there is the desire for the highest possible reactivity, i.e. a rapid crosslinking reaction in the adhesive layer after the joining process. This is of interest in particular if the adhesive bond is to be further processed as soon as possible after the joining. It is moreover advantageous to be able to carry out quality testing of the adhesive bonds as soon as possible after the joining process, since in this manner errors in the production process can be discovered faster and the reject rates in industrial production processes can thus be reduced significantly.
In the case of the commercially available 2-component adhesive dispersions of polyurethane dispersions and hydrophilically modified, predominantly aliphatic polyisocyanate, the producer of the adhesive bond must as a rule wait 2-3 days before he can carry out such quality testing.
One possibility for accelerating the crosslinking reaction in the adhesive layer lies in storage of the adhesive bond at elevated temperature (thermal curing). However, this is associated with high costs for generation of heat.
The present invention was therefore based on the object of providing adhesive systems based on aqueous formulations, which allow as long as possible a pot life to be achieved and at the same time already lead to adhesive bonds of high strength 24 hours after production of the adhesive bond and storage of the substrates glued in this way.
The object is achieved by the subject matter of the invention.